STRUCTURAL GEOLOGY AND GEOMORPHOLOGY OF THE NW COYOTE MOUNTAINS, ELSINORE FAULT ZONE, SOUTHERN CALIFORNIA

The Coyote Mountains are composed of pre-Cenozoic metamorphic basement rocks overlain by Miocene-Pleistocene sedimentary and volcanic rocks. The active dextral Elsinore Fault bounds the range on the SW.

The NW Coyote Mountains are marked by multiple ENE-striking, steeply SE-dipping sinistral reverse faults, the most prominent of which is the Canyon Sin Nombre Fault. On the northern flank of the range, this fault offsets Quaternary terrace surfaces. Along strike to the SW, this fault dramatically juxtaposes pre-Cenozoic basement to the SE against fluvio-deltaic sediments of the Pliocene-Pleistocene Palm Springs Group to the NW. Drag folding of the Palm Springs Group shows the sense of offset. At the mouth of Canyon Sin Nombre, this fault is truncated by the Elsinore Fault. To the NW, additional faults parallel the Canyon Sin Nombre Fault, offsetting and folding the Palm Springs Group and offsetting Quaternary terrace surfaces. We interpret the Canyon Sin Nombre Fault and parallel faults as rotated R’ faults related to the Elsinore Fault.

The most recent published map of the range shows the NE tip of the Canyon Sin Nombre Fault right stepping and curving 50° eastward to attain a SE strike and form the NE boundary of the range, juxtaposing the Palm Springs Group to the NE against the basement to the SW (Todd, 2004). However, our mapping demonstrates that the Canyon Sin Nombre Fault maintains its NE trend and that much of the SW margin of the Palm Springs Group is a conformable depositional contact with the marine Miocene-Pliocene Imperial Group. Moreover, the Imperial Group forms a buttress unconformity with the basement and with the volcanics and nonmarine sedimentary rocks of the Miocene Split Mountain Group. Thus only the SW margin of the range is bounded by a fault.

Along the SW range front we used geomorphic indices to analyze active tectonics related to the Elsinore Fault. These indices include stream length gradients, mountain front sinuosity, and drainage basin asymmetry. Geomorphic analysis of these indices demonstrates that the Coyote Mountains have experienced moderate uplift since the initiation of the Elsinore Fault, suggesting a lesser degree of tectonic activity in comparison to other regions of California affected by strike-slip faulting.

Todd, 2004, USGS Open File Report 2004-1363.